Method of improving scuff and cut resistance of ionomer covered game ball

ABSTRACT

Disclosed herein is a game ball and method for making the same. The game ball has an ionomeric cover which includes a combination of ionomeric crosslinks and covalent crosslinks. The game ball cover is superior in at least one of cut resistance and scuff resistance to a conventional cover that does not have covalent crosslinks but is otherwise substantially identical in composition. Crosslinks within the composition are achieved by irradiation. The game ball cover of the invention is particularly useful for improving the durability of golf balls to be struck with sharp-grooved clubs.

The present application is a continuation of U.S. application Ser. No.08/942,653 filed on Oct. 2, 1997 and issued on Apr. 6, 1999 as U.S. Pat.No. 5,891,973. In turn, U.S application Ser. No. 08/942,653 is acontinuation of U.S. application Ser. No. 08/595,898 which was filed onFeb. 6, 1996, and subsequently abandoned.

BACKGROUND OF THE INVENTION

This invention relates generally to game balls, and more particularly toa game ball, such as a golf ball, having an ionomeric cover.

Before the development of ionomers, balata was the preferred materialfor golf ball covers. Polyethylene also was proposed for use as a golfball cover material but was generally deemed highly inferior to balatain imparting playability and durability characteristics to the ball dueto its brittleness and high hardness, and thus never became acommercially successful golf ball cover material.

Balata golf ball covers have now been replaced to a great extent byionomeric cover materials. ionomers are copolymers of an olefin and ana, α, β-ethylenically unsaturated carboxylic acid with a portion of thecarboxylic acid groups neutralized by a metal ion. The metal ions serveas crosslinking agents, as they are ionically bonded to carboxylic acidgroups in adjacent copolymer chains. Instead of having thermallyirreversible covalent bonding, ionomers have thermolabile crosslinkingin which metal ions become part of the chemical structure of the ionomerupon crosslinking, and these crosslinks are reversible. For purposes ofthis application, this type of crosslinking is referred to as ioniccrosslinking. One of the advantages of ionic crosslinking in golf ballmaterials is the ability of ionic bonds to re-form after breaking as aresult of processing at elevated temperatures.

There are numerous advantages to the use of ionomers in making golf ballcovers. On the other hand, one drawback of conventional golf balls withsoft ionomeric covers are that the covers are prone to scuffing andcutting, particularly when hit with irons which have sharp grooves Itwould be useful to develop a golf ball with a soft ionomeric cover whichis highly resistant to cutting and scuffing by sharp-grooved clubs.

SUMMARY OF THE INVENTION

An object of the invention is to provide an ionomeric game ball coverhaving improved scuff resistance and/or cut resistance.

Another object of the invention is to provide a method for impartingimproved scuff resistance and/or cut resistance to a game ball cover.

Yet another object of the invention is to provide a golf ball with asoft cover which is well-suited for use with golf club irons havingsharp grooves.

Yet another object of the invention is to provide a method of forming agolf ball with a soft cover which has excellent scuff resistance and/orcut resistance.

Other objects will be in part obvious and in part pointed out more indetail hereinafter.

The invention in a preferred form is a game ball having a covercomprising an ionomer resin. The ionomer resin includes a copolymer ofan olefin and an α, β-ethylenically unsaturated carboxylic acid which isabout 10-100% neutralized with metal ions. The copolymer has asufficient degree of covalent crosslinking to impart to the coverimproved resistance to at least one of scuffing and cutting. In aparticularly preferred form of the invention, the game ball is a golfball. Preferably, the game ball has a dimpled surface. The covalentcrosslinking preferably comprises irradiation-induced covalentcrosslinking.

In a preferred form of the invention, the degree of covalentcrosslinking is appropriate to impart to the cover a Shore D hardnesswhich is no more than about 10% greater, and more preferably no morethan about 5% greater, than the Shore D hardness of a cover having anidentical composition but which does not include a substantial degree ofcovalent crosslinking. Preferably, the copolymer includes all acrylate.

Another preferred form of the invention is a method of treating a gameball. The method comprises the steps of obtaining a game ball having acover comprising an ionomer, and irradiating the ionomer in the coverunder conditions appropriate to covalently crosslink the ionomer inorder to increase the resistance of the cover to at least one ofscuffing and cutting without substantially impairing other playabilitycharacteristics of the ball. In a particularly preferred form of theinvention, the game ball is a golf ball. Preferably, the game ball has adimpled surface.

According to the preferred method of the invention, the game ball issubjected to electron beam treatment at a dosage of at least 2 megarads.The game ball cover preferably is irradiated prior to application of atop coat over a cover. The method of the invention preferably furtherincludes the step of applying a top coat over the cover before or afterirradiation.

The invention accordingly comprises the several steps and the relationof one or more of such steps with respect to each of the others and thearticle possessing the features, properties, and the relation ofelements exemplified in the following detailed disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. shows a golf ball according to the present invention.

FIGS. 2-3 schematically show one embodiment for practicing the method ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

The game balls of tie, present invention are surprisingly superior intheir scuff (abrasion) resistance and cut resistance to conventionalgame balls which have not been electron beam treated, and which containsimilar quantities of ionomer and have a similar hardness Furthermore,the golf balls and other game balls of the invention are comparable inscuff and cut resistance to game balls having non-ionomericcompositions, such as polyurethanes, with similar properties ofcompression, coefficient of restitution (COR) and hardness.

The game balls of the invention are formed by first obtaining anuncoated or coated game ball having an ionomeric cover. An “uncoated”game ball as the term is used in this application is a one, two, ormulti-piece game ball to which no primer or top coat has been appliedover the ionomeric cover. In contrast, a “coated” game ball as this termis used in this application is a ball which has a primer coat and/or atop coat over the ionomeric cover layer. The coated or uncoated gameball of the invention is subjected to irradiation under conditionsappropriate to induce covalent crosslinking of the ionomer. This type ofdirect covalent bonding has been found to take place in ionomeric covermaterials when electron beam treatment is applied at a dosage of 2 ormore megarads and is expected to also be useful at lower dosages, forexample, 1 megarad.

For clarity of description and ease of understanding, the invention willbe described in connection with golf balls although it will beunderstood that other game balls, including but not limited tosoftballs, basketballs, baseballs, soccer balls, volleyballs, streethockey balls, footballs, and the like, can advantageously employ thefeatures of the present invention.

Referring now to the drawings, and in particular to FIG. 1, a golf ballaccording to the present invention is shown and is designated as 8. Theball has a core 10, which is solid, or is formed from any other suitabletype of core composition. An Monomeric cover 12 surrounds the core 10. Athin primer coat 14 is applied to the outer surface of cover 12. A thintop coal 16 surrounds the primer coat 14. The thicknesses of primer coat14 and top coal 16 are exaggerated for illustrative purposes.

In accordance with the present invention, after the cover layer 12 isapplied over the core, the cover layer 12 is subjected to irradiation ata dose of about 1 or more megarads in order to covalently crosslink theionomeric cover material. Particularly good results are obtained whenthe dosage is 2-12 megarads. In a most preferred form of the invention,a dosage of 4-8 megarads is utilized. As used herein, the term“irradiation” refers to short-duration irradiation using an electronbeam or the like, rather than to mere exposure to sunlight, which wouldresult in a dosage of well below 1 megarad. Irradiation takes place at atemperature below the melting or deformation temperature of the coverlayer, and for convenience preferably takes place at ambienttemperature.

The cover 12 can be irradiated prior to or after application of primercoat 14 and top coat 16. Furthermore, primer coat 14 can be eliminatedif adhesion of top coat 16 to cover 12 is sufficient to render the ballsuitable for competitive play, as is commonly the case with softballsand baseballs, and may also be the case for other game balls.

The game ball of the invention can be irradiated with electrons,neutrons, protons, gamma rays, x-rays, helium nuclei, or the like. In aparticularly preferred form of the invention, the scuff and cutresistance of cover 12 is enhanced by subjecting the cover to electronbeam treatment at a dosage sufficient to significantly improve scuffresistance and COR without excessively hardening the compression. Thegame ball preferably obtains an improvement in COR of at least 0.5% as aresult of irradiation of the cover.

The cover composition preferably contains high quantities of ionomer.Thus, the irradiated cover material has a combination of ioniccrosslinks and covalent crosslinks. Particularly preferred ionomers orionomer blends include ionic copolymers containing an olefin, anunsaturated carboxylic acid, and an acrylate. Such polymers typically,although not necessarily, have a Shore D hardness in the range of 20-60.Non-ionomeric materials can be blended with the ionomer as long as anacceptable increase in scuff resistance and/or cut resistance isobtained as a result of covalent crosslinking of the ionomer.Non-limiting examples of materials to be blended with an ionomer includeethylene-ethyl acrylate, ethylene-methyl acrylate, ethylene-vinylacetate, low density polyethylene, linear low density polyethylene,metallocene catalyzed polyolefins such as ENGAGE polyolefins availablefrom Dow Chemical and EXACT polyolefins available from Exxon,non-ionomeric acid copolymers such as PRIMACOR, available from DowChemical, and NUCREL, available from Du Pont, and a variety ofthermoplastic elastomers, including KRATON, available from Shell,SANTOPRENE, available from Monsanto, and HYTREL, available from DuPont,etc.

If the game ball cover is irradiated prior to application of a primercoat 14 and/or top coat 16, there is no particular restriction on thetype of primer coat and/or top coat to be used. If irradiation occursafter application of a primer coat and/or a top coat over the cover 12,it is important to ensure that the radiation will penetrate the coatingand that the dosage of radiation is sufficient to covalently crosslinkthe cover material without adversely affecting the properties of theprimer and/or top coat to a substantial degree. Due to the thinness ofthe primer coat and top coat on most game balls, including golf balls,it has been found that little change in radiation dosage is required ifirradiation occurs after application of such coatings.

Golf balls according to the invention preferably have a post-irradiationPGA compression of 10-110. In a particularly preferred form of theinvention, the golf balls have a PGA compression of about 40-100 afterirradiation. It has been found that excellent results are obtained whenthe post-irradiation PGA compression of the golf balls is 60-100. Theirradiation method of the invention preferably results in an increase inPGA compression of at least 5% as compared to the PGA compression of theball prior to treatment. The coefficient of restitution of the golfballs of the invention after treatment is in the range of 0.780 orgreater. Preferably, the COR of the golf balls is in the range of0.790-0.830 and most preferably 0.800-0.830. The Shore D hardness of thegolf balls of the invention after irradiation is in the range of 40-80.Particularly good results are obtained when the Shore D hardness of thegolf balls is in the range of 50-70, and most preferably 50-60.

The invention is particularly well suited for use in making dimpled,pebbled, and other game balls which do not have a smooth outer surface,including game balls with simulated stitching. A smooth ball is lesssusceptible to scuffing than a dimpled ball since dimples give the golfclub groove a sharp-edged surface to “catch.” Pebbles clearly aresusceptible to shearing when dribbled on a hard surface, or the like.Likewise on a molded-cover softball, the stitching is a raised area thatwill be sheared or compressed more than the smooth-surfaced area by abat and/or by the turf, dirt, etc.

One embodiment of a method for electron beam treating golf ballsaccording to the invention can be described in connection with FIGS. 2and 3. The golf ball 8 is placed on a channel 18 along which it slowlymoves. Electrons 20 from electron beam lamps 22 contact the surface ofthe ball 8. The lamps are positioned to provide a generally uniform doseof radiation on the entire surface of the ball as the ball rolls alongthe channel 18. Preferably, the balls are irradiated with an electronbeam dosage of 1 or more megarads, more preferably 2-12 megarads. Theintensity of the dosage preferably is in the range of 1-20 MeV.

The golf balls of the invention are found to exhibit a post-treatmentscuff resistance in the range of 1-3 on a scale of 1-4. It is preferredthat the treatment be appropriate to provide the golf balls with a scuffresistance of 1-2.5, and more preferably 1-2. Golf balls according tothe invention have a cut resistance in the range of 1-3 on a scale of1-5. It is preferred that the golf balls of the invention have a cutresistance of 1-2.5 and most preferably 1-2.

The scuff resistance test was conducted in the following manner: aTop-Flite Tour pitching wedge (1994) with box grooves was obtained andwas mounted in a Miyamae driving machine. The club lace was oriented fora square hit. The forward/backward tee position was adjusted so that thetee was four inches behind the point in the downswing where the club wasvertical. The height of the tee and the toe-heel position of the clubrelative to the tee were adjusted in order that the center of the impactmark was about ¾ of an inch above the sole and was centered toe to heelacross the face. The machine was operated at a clubhead speed of 125feet per second. Three samples of each ball were tested. Each ball washit three times. After testing, the balls were rated according to thefollowing table:

Rating Type of damage 1 Little or no damage (groove markings or dents) 2Small cuts and/or ripples in cover 3 Moderate amount of material liftedfrom ball surface but still attached to ball 4 Material removed orbarely attached

Cut resistance was measured in accordance with the following procedure:A golf ball was fired at 135 feet per second against the leading edge ofa 1994 Top-Flite Tour pitching wedge, wherein the leading edge radius is{fraction (1/32)} inch, the loft angle is 51 degrees, the sole radius is2.5 inches, and the bounce angle is 7 degrees.

The cut resistance of the balls tested herein was evaluated on a scaleof 1-5. A 5 represents a cut that extends completely through the coverto the core; a 4 represents a cut that does not extend completelythrough the cover but that does break the surface; a 3 does not breakthe surface of the cover but does leave a permanent dent; a 2 leavesonly a slight crease which is permanent but not as severe as 3; and a 1represents virtually no visible indentation or damage of any sort.

It has been found that golf balls which are treated according to theirradiation technique of the present invention exhibit a particularimprovement in scuff and/or cut resistance. This improvement isparticularly significant when the golf balls are struck with asquare-grooved iron. It is has been found that square-grooved irons andother sharp-grooved irons tend to abrade and damage golf ball coversmore readily than irons having “V-type” grooves.

Having generally described the invention, the following examples areincluded for purposes of illustration so that the invention may be morereadily understood and are in no way intended to limit the scope of theinvention unless otherwise specifically indicated.

EXAMPLE 1

Polybutadiene golf ball cores having an average coefficient ofrestitution (COR) of 0.790 and a PGA compression of 90 were obtained.The cores were covered with a relatively soft, 0.055 inch thick coverformed from an ionomeric composition, designated as ionomer 1, whichincludes a blend of a hard sodium ionomer and a soft zinc ionomer.Pigments and an optical brightener were also included in the covercomposition. The hard ionomer is a copolymer containing two mcnomertypes, namely an α-olefin and an acrylic-type carboxylic acid. The softionomer is a copolymer which contains three types of monomers, namely anα-olefin, an acrylic-type carboxylic acid, and an acrylate. The coverwas formed over the core The balls were primed with a polyurethane-basedprimer with a thickness of about 0.5 thousandths of an inch and coatedwith a polyurethane top coat with a thickness of about 0.5 thousandthsof an inch. The properties of these balls prior to electron beamtreatment are shown on Table 1.

A portion of the balls were subjected to electron beam treatment atdosages of 2, 4, 6, 8 and 11 megarads at an energy level of 10 MeVChanges in the properties of the balls are shown on Table 1.

As indicated on Table 1, the scuff resistance of the golf ballssubstantially improved between electron beam dosages of 2 and 6megarads. The cut resistance of the balls improved between electron beamdosages of 6 and 8 megarads. Meanwhile, the change in Shore D hardnessin the dosage range of 2 to 8 was only 1.

EXAMPLE 2

Golf ball cores having an average PGA compression of 85 and an averageCOR of 0.800 were obtained. The cores were covered with a hardzinc-sodium Monomeric cover composition containing two monomer types,namely an α-olefin and a carboxylic acid. The initial properties of thecovered cores prior to priming and finish coating are shown on Table 1.

A portion of the golf balls were primed with the same polyurethane-basedprimer as was used in Example 1 and then coated with the samepolyurethane top coat as was used in Example 1. Subsequently, a portionof both the unprimed and unfinished golf balls and the primed andfinished golf balls were subjected to electron beam treatment in thedosages shown on Table 1. Properties of compression, COR, Shore Dhardness and scuff resistance are shown on Table 1.

As indicated in Table 1, the scuff resistance of the finished golf ballsincreased substantially while resulting in a minimal increase in Shore Dhardness.

As will be apparent to persons skilled in the art, various modificationsand adaptations of the structure above described will become readilyapparent without departure from the spirit and scope of the invention,the scope of which is defined in the appended claims.

TABLE 1 Cover Dosage PGA Scuff Cut Material (Megarads) Weight Comp. CORShore D Resistance Resistance Ionomer Blend 1 2.0 45.5 93 .788 54 2.52-3 (coated prior to 4.0 45.5 97 .797 55 1.2 2-3 treatment) 6.0 45.5 98.795 55 1.5 2-3 8.0 45.5 98 .797 55 1.5 2 11.0 45.5 101 .802 55 1.5 2 045.5 92 .787 54 3.5 3 Ionomer Blend 2 2.0 45.4 96 .822 63 — — (uncoated)4.0 45.4 97 .822 63 — — 0 45.4 88 .812 62 — — Ionomer Blend 2 6.0 45.5101 .829 64 1.5 1-2 (coated with 8.0 45.5 103 .828 64 1.5 1-2 primer andtop 0 45.5 88 .813 63 2.0 1-2 coat prior to treatment)

What is claimed is:
 1. A method of forming a game ball comprising:obtaining a game ball having a cover comprising an ionomer wherein theionomer includes a copolymer of an α-olefin, an acrylate, and anα,β-ethylenically unsaturated carboxylic acid which is about 10-100%neutralized with metal ions, and irradiating the ionomer in the coverunder conditions appropriate to covalently crosslink the ionomer inorder to increase the resistance of the cover to at least one ofscuffing and cutting without substantially impairing other playabilitycharacteristics of the ball.
 2. A method according to claim 1 whereinthe game ball is a golf ball.
 3. A method according to claim 1, whereinthe game ball has a dimpled surface.
 4. A method according to claim 1,wherein the cover is irradiated using an electron beam.
 5. A methodaccording to claim 4, wherein the cover is subjected to electron beamtreatment at a dosage of at least 2 megarads.
 6. A method according toclaim 1, wherein the game ball cover is irradiated after application ofa coating over the cover.
 7. A method according to claim 1, furthercomprising the step of applying a top coating over the cover afterirradiation.
 8. A method of making a game ball, comprising obtaining anunfinished game ball having a cover comprising an ionomer wherein theionomer includes a copolymer of an α-olefin, an acrylate, and anα,β-ethylenically unsaturated carboxylic acid which is about 10-100%neutralized with metal ions, forming a coating over the cover, andirradiating the ionomer in the cover at a dosage of at least about 2megarads to covalently crosslink the ionomer to a degree sufficient toincrease at least one of the scuff resistance and cut resistance of thecover without reducing the coefficient of the restitution of the cover.9. A method according to claim 8, wherein the game ball is a golf ball.10. A method according to claim 8, wherein the game ball has a dimpledsurface.
 11. A game ball having a cover wherein said cover comprises atleast one ionomer resin that is a copolymer of an olefin, an acrylateand an α,β-ethylenically unsaturated carboxylic acid, wherein saidcopolymer is 10 to 100% neutralized with metal ions and wherein saidcopolymer has a sufficient amount of radiation-induced covalent crosslinking to increase the resistance of the cover to at least one ofscuffing and cutting and to impart to the cover a Shore D hardness whichis at most 10% greater than the Shore D hardness of a cover having asubstantially identical composition but which does not have the amountof covalent crosslinking as from the radiation-induced covalentcrosslinking.
 12. A game ball according to claim 11, wherein the gameball is a golf ball.
 13. A game ball according to claim 11, wherein thecover has a dimpled surface.
 14. A game ball according to claim 11,wherein the cover has a Shore D hardness of 65 or less.
 15. A game ballaccording to claim 11, wherein the degree of covalent crosslinking issufficient to impart to the ball a coefficient of restitution which isat least 0.50% greater than a coefficient of restitution of a ball witha cover having a substantially identical composition but which does nothave radiation-induced covalent crosslinking.
 16. A game ball accordingto claim 11, wherein the degree of covalent crosslinking is sufficientto impart to the ball a PGA compression which is at least 5% harder thanthe PGA compression of a golf ball with a cover having a substantiallyidentical composition but which does not have a substantial degree ofcovalent crosslinking.
 17. A game ball comprising a cover wherein saidcover comprises at least one ionomer resin that is a copolymer of anolefin, an acrylate and an α,β-ethylenically unsaturated carboxylicacid, wherein said copolymer is 10 to 100% neutralized with metal ionsand wherein said copolymer has a sufficient amount of radiation-inducedcovalent cross linking to increase the resistance of the cover to atleast one of scuffing and cutting while resulting in a cover Shore Dhardness which is not more than about 0.5% higher than the Shore Dhardness of a cover having a substantially identical composition butwhich does not have the amount of covalent crosslinking as from theradiation-induced covalent crosslinking.
 18. A game ball according toclaim 17, wherein the game ball is a golf ball.
 19. A game ballaccording to claim 18, wherein the cover has a dimpled surface.
 20. Agame ball according to claim 17, wherein the cover has a Shore Dhardness of 65 or less.
 21. A game ball according to claim 17, whereinthe degree of covalent crosslinking is sufficient to impart to the balla coefficient of restitution which is at least 0.50% greater than acoefficient of restitution of a ball with a cover having a substantiallyidentical composition but which does not have radiation-induced covalentcrosslinking.
 22. A game ball according to claim 17, wherein the degreeof covalent crosslinking is sufficient to impart to the ball a PGAcompression which is at least 5% harder than the PGA compression of agolf ball with a cover having a substantially identical composition butwhich does not have a substantial degree of covalent crosslinking.